The present invention relates to a polyvinyl acetal sponge sheet having an improved wiping performance of water and, more particularly, to a polyvinyl acetal sponge sheet which is suited for use as a waste for removing water droplets on the surface of the car body and wiping the surface of a precision machine, machinery components, instrument or the like, and which is suited for wiping the human body.
A sponge made of a polyvinyl acetal porous elastic material has hitherto been used as a material for wiping water. Particularly, a sheet-shaped sponge made of a polyvinyl acetal porous elastic material, or a sheet-shaped article obtained by interposing reinforcing clothes between polyvinyl acetal porous elastic materials and integrating them into a composite has good water absorption capability and, therefore, it is employed as a wiping material for wiping to remove water droplets, dust, etc. on the surface of an article having comparatively good flatness. That is, it has already been commercialized as a waste for removing water droplets on the surface of the car body after car washing and wiping the surface of a precision machine, machinery components, instrument or the like.
However, in the case of coating the car body, several coats of paint have recently been applied on a rust resisting coat so as to obtain a beautiful body color. In a metallic coating, particularly, a clear coat of paint as a top coat layer is applied on a metallic layer, resulting in coat configuration of five or more layers including a prime coat.
This clear coat is liable to be scratched because it is comparatively soft, and often take fine scratches according to a manner of wiping water after car washing. Sometimes, in a severe case, luster of the coat is lost. For example, when the water absorption capability of a wiping cloth is insufficient, water can not be wiped completely at a time and the surface of the car body must be rubbed several times, thereby taking scratches. When unwiped water droplets are left on the coated surface, the water droplets cause the same action as that of convex lens on exposure to sunlight in a summer season, thereby burning the coated film, resulting in blotch.
Accordingly, when the car body is wiped after car washing, a flexible one having a soft surface is preferably used so as not to take scratches, and more preferred one has good water absorption properties and does not leave any unwiped portions after wiping. It is also important that the car body is not wetted by retaining once absorbed water in a material, that is, water retention properties are enhanced.
By the way, the reason why the polyvinyl acetal sponge sheet is used for wiping water as described above is as follows. That is, since the polyvinyl acetal sponge sheet has not only a larger water absorption capability than that of a conventional towel, but also a capability of vigorously sucking up water, water droplets can be removed only by contacting with them and it is not necessary to rub strongly like the towel.
Therefore, the present inventors have considered to make the sponge layer dense so as to enhance water absorption properties and water retention properties. However, it has been found that, although the above-mentioned polyvinyl acetal sponge sheet is generally used after previously dipping it in water and properly squeezing the wet sheet, when the sponge layer is made dense, frictional resistance and squeak on use are increased, thereby causing a problem such as low working factor.
Under these circumstances, the present inventors have intensively studied. As a result, they have found that a polyvinyl acetal sponge sheet having a specific structure can reconcile the water absorption properties and working factor on wiping after car washing, thus completing the present invention. An object of the present invention is to provide a sponge sheet suited for use as a material for car washing which does not leave unwiped portions during wiping after car washing and has a small frictional resistance and a high working factor.
The present invention provides a polyvinyl acetal sponge sheet comprising a polyvinyl acetal sponge layer on at least the surface thereof, and a film on at least part of the surface of said polyvinyl acetal sponge layer.
In the sponge sheet of the present invention, a pore opening percentage of the film is preferably from 10 to 60% and, more preferably, the polyvinyl acetal sponge layer is made of a polyvinyl acetal resin having an acetalization degree of 50 to 80% and is composed of a porous structure having fine continuous pores with an average pore diameter of 10 to 160 xcexcm and a porosity of 80 to 95%.
The polyvinyl acetal sponge in the present invention refers to a porous structure having fine continuous pores which is obtained by a crosslinking polyvinyl alcohol and a pore-forming agent with aldehydes in the presence of a catalyst and removing the pore-forming agent.
According to one embodiment of the present invention, the water retention capability of the sponge can be enhanced by forming a film on the surface of the sponge. According to a second embodiment of present invention, there can be obtained a sponge sheet which is free from a squeak feel because the pore opening percentage of the film is from 10 to 60%, and which has high water retention capability. That is, when the pore opening percentage is smaller than 10%, the squeal feel on operation becomes strong. On the other hand, when the pore opening percentage exceeds 60%, the water retention capability is lowered and, therefore, once retained water is liable to spill on the coated surface, thereby leaving the unwiped portions. The term xe2x80x9cpore opening percentagexe2x80x9d of the film in the present invention refers to a proportion of pores, which constitute the structure of the porous material as the lower layer, exposed on the surface film layer on the sponge layer, in terms of a surface ratio. Pore opening percentage (%)=(are wherein pores are exposed)/(total surface area of porous material)xc3x97100.
The pore opening percentage of 100% means the state where no film exists on the surface of the porous material, while 0% means the state where the whole surface area is covered with the film.
According to the invention of claim 3, there can be obtained a sponge sheet having high flexibility and high water absorption properties. That is, the polyvinyl acetal sponge in the present invention has an acetalization degree of 50 to 80%. When the acetalization degree exceeds 80%, the resulting sponge is not suited for use because it is also hard on wetting with water.
An average pore diameter of the sponge sheet of the present invention is from 10 to 160 xcexcm. When the average pore diameter is smaller than 10 xcexcm, the squeak feel on operation becomes strong. On the other hand, when the average pore diameter exceeds 160 xcexcm, the water absorption properties become worse and the water retention capability is lowered and, therefore, once retained water is liable to spill on the coated surface, resulting in low working factor.
A porosity of the sponge sheet of the present invention is from 80 to 95%. When the porosity is smaller than 80%, the above-mentioned water retention capability is lowered. On the other hand, the porosity exceeds 95%, the strength of the sheet is lowered, which is not preferred.
The sponge sheet of the present invention can be produced by the following manner. That is, the sponge sheet can be obtained by reacting a stock solution, which is prepared by mixing polyvinyl alcohol, aldehydes, acids and a fine powder of starches, in the state of being partitioned with a paper in a layered configuration, removing the unreacted aldehydes, acid sand fine powder of starch through washing with water after the completion of the reaction, and separating at the portion of the above-mentioned paper as a separator.
The polyvinyl alcohol used in the present invention may be completed or partially saponified one having a polymerization degree of 300 to 2000, wherein part of hydroxyl groups is substituted with the other functional group, and these polyvinyl alcohols can be used alone or in combination thereof.
The aldehydes used in the present invention act as an aliphatic or aromatic crosslinking agent capable of converting polyvinyl alcohol into polyvinyl acetal, which may also be monohydric or polyhydric. Particularly, formaldehyde and acetaldehyde are preferred to afford physical properties as the polyvinyl acetal porous material.
The salts used in the present invention are those acting as a catalyst and there can be used, for example, mineral acids such as hydrochloric acid, sulfuric acid, nitric acid, etc.; organic acids such as oxalic acid, citric acid, maleic acid, etc.; or salts of a strong acid and a weak base, which are acidic in an aqueous solution.
The fine powder of starches used in the present invention refers to a granular material (starch granules) obtained by separation/extraction of plants, and plants from which starched are derived include, for example, grains such as rice, wheat, corn, etc.; and corns such as potato, sweet potato, taro, etc. A so-called modified starch powder obtained by incorporating a hydrophobic functional group into hydroxyl groups of these starch granules may also be used. This fine powder of starches acts as a pore-forming agent in the production of the polyvinyl acetal porous material, and the pore diameter and porosity can be adjusted by changing the kind and amount of the starch.
The paper used in the present invention is used as a separator, and is preferably a paper which is completely disintegrated and dispersed after the completion of the reaction and which does not contain a binder, an additive or the like. In order to obtain a sponge sheet having a predetermined pore opening percentage, those having fixed water penetration properties are used. That is, those having a water penetration time required to penetrate 500 ml of water through a paper having a fixed area within a proper range are used. When the water penetration time is long, the pore opening percentage of the sponge sheet becomes small. On the other hand, when the water penetration time is short, the pore opening percentage becomes large.